As methods for improving the quality and physical properties of polyester films, polymer blend techniques are actively examined in recent years. Especially studies concerning the blends consisting of a liquid crystal polyester and a non-liquid crystal polymer are actively conducted worldwide, and many inventions are disclosed as can be seen in U.S. Pat. No. 4,386,174, International Publication No. W087-05919, U.S. Pat. No. 4,728,698, Japanese Patent Laid-Open (Kokai) No. 57-25354, Japanese Patent Laid-Open (Kokai) No. 5-169527, etc.
In general, since a liquid crystal polyester has a high Young's modulus, it can be finely dispersed in a polyester film, to reinforce the polyester film. As another utilization method, the high flowability of a liquid crystal polyester is used. Since a liquid crystal polyester has an effect to improve the flowability of a polymer, hence to control the shear heat generated in the extrusion process, blending it with a non-liquid crystal polyester is effective for decreasing the thermal decomposition and gelation product and oligomer in the film, for improving the quality of the polyester film.
It is reported in U.S. Pat. No. 4,728,698, etc. that if a liquid crystal polyester exists as spherical domains in a non-liquid crystal polyester, the Young's modulus cannot be enhanced, but that if a liquid crystal polyester exists as fibers, the Young's modulus can be enhanced remarkably. However, if a liquid crystal polyester exists as fibers in a film like this, there is a problem that it is difficult to enhance the Young's modulus in the direction perpendicular to the fiber-oriented direction, though the Young's modulus in the fiber-oriented direction can be enhanced. For example, in a conventional polyester film with fibrously dispersed domains, the liquid crystal polyester fibers are mostly oriented in the machine direction of the film, and in this case, though the Young's modulus in the machine direction of the film is remarkably enhanced, the Young's modulus in the transverse direction of the film is not enhanced. Furthermore, if the bonding at the interfaces between a non-liquid crystal polyester and a liquid crystal polyester is insufficient, excessive molecular orientation by stretching forms many voids, to degrade the clarity and to cause film breakage disadvantageously. A polyester film with a liquid crystal polyester dispersed in layers or sheets for improving the gas barrier property is disclosed in Japanese Patent Laid-Open (Kokai) No. 5-169527. The present inventors produced polyester films according to the method stated in the examples of the patent, and measured the Young's moduli of the obtained films, but could not obtain a high value as achieved in the present invention.
The general tendency in the formation of a polyester film that if the degree of molecular orientation is enhanced to improve the Young's modulus, the heat shrinkage increases when a liquid crystal polyester is blended.
If a blend obtained by adding a liquid crystal polyester to a non-liquid crystal polyester is formed into a film, the clarity of the film is degraded disadvantageously since the domain size of the liquid crystal polyester dispersed in the polyester film is as large as, or larger than the wavelengths (400 to 900 nm) of visible light. That is, if the domain size of the dispersed liquid crystal polyester is large, the film surface is heavily roughened irrespective of whether the domains are spherical, oblate, fibrous, needle-like or laminar. So, it is difficult to use the film as a base film for a magnetic tape, etc., and for this application, a polymer layer with a smooth surface must be laminated on a polyester film containing a liquid crystal polyester disadvantageously. The degradation of clarity and surface smoothness described above becomes more remarkable if the liquid crystal polyester content of the polyester film is increased for improving the Young's modulus and other quality.
The object of the present invention is to solve the above mentioned problems of the prior art, and to provide a high quality polyester film excellent in mechanical properties, low heat shrinkage, clarity, surface properties (surface smoothness, surface slipperiness and abrasion resistance), long-term thermostability and electric properties, less surface defects and low in oligomer content.